This invention relates to a method of directly bonding metals to substrates such as a ceramic or a metal and, more particularly, to a method of performing the bonding operation in an inert atmosphere.
The general subject of bonding metals to ceramics or to other metals is not new. Such processes have been of interest to many industries for some time. For example, spark plug manufacturers have long faced the problem of bonding ceramic insulators to metallic spark plug bases.
Various methods of bonding nonmetallic members to metallic members have been employed in the past. One exemplary method includes the application of a mixture of titanium hydride and a solder metal, such as copper, silver, or gold, to the member to be metallized or bonded. Next, the hydride is disassociated by the application of heat in the presence of the solder metal. In this process, the heating is preferably done in a non-oxidizing atmosphere, such as pure dry hydrogen. The description found in U.S. Pat. No. 2,570,248 is typical of such a process.
Another method of bonding metals to ceramics is described by J. T. Klomp of Philips Research Laboratories. This method is described as employing low-oxygen affinity metals applied to a ceramic under high pressures, e.g., 1 Kg/cm.sup.2. Where oxygen-affinity metals are employed, sufficiently high pressures are required "to destroy the oxide film so that metal-ceramic contact can be made." Hence, this method employs extremely high pressures to effect bonding. While these methods may produce desirable bonds for many applications, obviously the most desirable bonding system would be a direct bond between the copper and the ceramic substrate which did not require high pressures to form.
Another process for forming metallic bonds is described in U.S. Pat. No. 2,857,663 by James E. Beggs. Basically, this method employs an alloying metal, such as metal from the titanium group, IVb of the Periodic Table, and an alloying metal, such as copper, nickel, molybdenum, platinum, cobalt, chromium or iron. When the alloying metal and a member of the titanium group are placed between non-metallic refractory materials or a non-refractory metallic material and a metallic material and are heated to a temperature at which a eutectic liquidus is formed, a strong bond forms between the adjacent members. While this process has been satisfactory for many applications, the desire to improve the integrity of the bond, increase the thermal conductivity between a metal member and a non-metallic refractory member as well as provide a high current carrying conductor on the non-metallic refractory member has prompted researchers to seek still other methods of bonding metals to nonmetals.
The formation of bonds between two metallic members has been achieved in various ways. For example, certain metals can be bonded together with the use of solders. Other metals are bonded together by welds, such as arc welds or spot welds. Where certain metals can not be directly bonded to each other, generally intermediate metallic members are used to form the bond. The foregoing methods are frequently not compatible with demanding applications, such as integrated circuit fabrication, or, if compatible, are frequently economically unacceptable.
More recently, methods of bonding metals to metals and metals to ceramics have been developed utilizing only a eutectic of the metal and a gas as a bonding medium. Descriptions of these methods will be found in U.S. Pat. Nos. 3,744,120 (Burgess, et al) and 3,766,634 (Babcock, et al), both assigned to the assignee of the present invention. The metal-gas eutectic process, as taught in the referenced patents, involves placing the metal to be bonded on the ceramic or metal substrate. The combination is then heated in the presence of a reactive gas to a temperature below the melting point of the metal but sufficiently high that the eutectic is formed between the metal and the gas. While this method has proven successful for certain applications, further improvement was desired. One reason improvement was desired is that there is a reaction between the gas and the metal. For particularly demanding applications, this may cause a problem. For example, when bonding copper to ceramics for use in hybrid electronics circuits, the use of the reactive atmosphere causes a coating of copper oxide to be formed on the copper, thus sometimes necessitating an oxide removal step.
It is, therefore, an object of this invention to provide a method for directly bonding metals to substrates of ceramic or metal which overcomes the aforementioned disadvantages of the prior art.